Structure for supporting filaments in vacuum tubes



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D. SLOAN 2,405,762 STRUCTURE FOR SUPPORTING FILAMENTS IN VACUUM TUBESOriginal Fi1e d-Nov. 4, 1940 10 Sheets-Sheet 4 II I U m N A K I S W/ mwMM W 0 w w A 0 m o 7 v Aug. 13, 1946; D. H. SLOAN STRUCTURE FORSUPPORTING FILAMENTS IN VACUUM TUBES Original Filed Nov/ 4, 1940 Y "10Sheets-Sheet 5 INVENTOA 0.4 wo H. SLOA/V.

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STRUCTURE FOR SUPPORTING FILAMENTS IN VACUUM TUBES OriginalFiIed NQV. 4,1940 10 SheetsSheet s v INVENTOR.

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STRUCTURE FOR SUPPORTING FILAMENTS-IN VACUUM ,TUBES Aug. 13, 1946. D. H.SLOAN Original Filed NOV 4, 1940 v 10 Sheets-Sheet 8 INVENTOR. DAV/D H.$404M.

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Aug. 13, 1946. 0.1-1. SLOAN 2,405,762

STRUCTURE FOR SUPPORTING FILAMENTS IN vAcUU TUBES Original Filed Nov. 4,1940 10 Sheets-Sheet 9 ,7 57 s 7 I j as.

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DAVIDH$LOAM BY um TUBES Au 1s, 1946. D. H. sLoAN URE FOR su PoRTINGFILAMENTS VACU S TRUCT Original-Filed Nov. 4, 1940- 10, Sheets-Sheet 1oDA v/b H. $10M.

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Patented Aug. 13, 1946 STRUCTURE FOR SUPPORTING FILAMENTS IN VACUUMTUBES David H. Sloan, Berkeley, Calif, assignor to Research Corporation,New York, N. Y., a corporation of New York Original application Nov364,284. Divided and 1941, Serial No. 397,234

24 Claims. 1

This invention relates to electronic tubes, and particularly to tubesadapted for the production and modulation of ultra-high frequencyoscillations, i. e., oscillations of frequencies of the order of 1,000megacycles, This application is a division of my prior applicationSerial No. 364,284, filed November 4, 1940.

The. progress of electronic and radio development since the inception ofthe art has been marked by two steady advances. One of these advanceshas been toward higher power, the ther toward higher frequencies. Thelatter line of advancement has been, to a certain extent at least,incompatible with the first, since with increasing frequency the effectof interelectrode capacity has become greater and more troublesome.Nevertheless, up to the last few years, the dithculties have been met bya steady evolutionary process consisting in large degree of refinementin detail, which has enabled the vacuum tube art to keep pace with theincreasingly rigid demands of the manufacturers and operators oftransmitting and receiving apparatus.

The attempt within recent years to carry the useful spectrum into therange of wavelengths in the range of a meter and less has involveddifficulties of a new order of magnitude. For one thing, the frequenciesinvolved are so high that the transit time of an electron stream acrossthe interelectrode spaces of the tubes becomes an appreciable fractionof a cycle. For another, even with connecting leads reduced to minimumlengths, their inductance has been sufficient so that th capacitiesrequired in tuning them to the desired frequencies are mall incomparison with the interelectrode capacities inconventional tubestructures and these capacities have therefore become not merely anuisance, limiting the efficiency of operation, but frequently anabsolute bar to such operations; so much so, in fact, a

that it has been only with tubes of very small size and consequent smallpower output that operation has been obtainable at all.

There therefore exists at the present time a need for a tube which willmeet the severe requirements of producing large power outputs bygeneration or amplification at extremely high frequencies. Theserequirements are first, a cathode-grid structure which will efiectivelymodulate an electron stream without the application of excessive controlvoltages; second, a cathodegrid structure whose capacityand inductancerelationships are so proportioned that they may be tuned to the highoperating frequencies desired; third, a structure lending itself tocircuits of low ember 4, 1940, Serial No. this application June 9,

relative radio-frequency resistance of high impedance, so that excessiveenergy will not be required to swing them through the necessary range ofcontrol voltages; fourth, fixed relationship between the variouselements, irrespective of temperature or ordinary shock, so that thefrequency to which the device as a whole is tuned will not be affectedby relative changes of position; fifth, minimum undesired or incidentalradiation from the various elements of the tube and its auxiliaries;sixth, a minimum of insulating material subjected to high-frequencyfields. To these may be added the secondary requirements ofdemountability for replacement of filaments, facility in water coolingand avoidance of hot spots, and ease of tuning.

From the conventional approach these requirements are incompatible to alarge degree. A high degree of control requires close spacing of cathodeand grid, which leads to high interelectrode capacity. Rigid structureordinarily means massive structure, which again leads to highinterelectro'de capacity. Water cooling systems tend to form effectiveantennae, leading to large stray power radiation. The broad, purpose ofmy invention is therefore to reconcile these and other apparentincompatibles.

Pursuant to this general purpose, among the objects of this inventionare: To provide a tube which is capable of producing many kilowatts ofpower at extremely high frequencies; to produce a high frequencygenerator of great frequency stability; to produce a high frequencyamplifier and oscillator tube of relatively high efficiency, andparticularly to produce such a tube wherein the losses due to undesiredradiation from the tube itself are reduced to negligible proportions; toproduce a high frequency oscillator and amplifier which may be tuned tooperate at any desired frequency throughout a reasonably wide range; toprovide a high frequency oscillator and amplifier which may beconstructed with the high degree of accuracy required to meet the closetolerances demanded by the frequency of operation and to maintain thosetolerances under the changes of temperature Produced by such operation;to provide an electronic tube of the character described which may befully fluid cooled and wherein the cooling system does not introducematerial parasitic radiation of radio-frequency power; to provide a highpower oscillator and amplifier tube which is readily demountable forreplacemnt of filaments; to provide means of density-modulating anelectron stream at ultra-high frequencies, in

order to produce extremely short bursts of electron emission occurringat the peaks of the oscillation and of substantially uniform velocity,whereby the conversion of energy into high frequency power occurs athigh efiiciency; to provide a type of structure for high frequencyelectronic tubes which is of great flexibility, and which will, becauseof such flexibility, permit the construction of tubes exactly adapted toa large variety of powers and services; to provide a novel and efiectivemethod of tuning apparatus of the character described; and to provide atype of electrode support for high frequency A electronic devices whichis massive and rugged, and which, at the same time, does not introduceinterelectrode capacities which either severely limit the frequenciesupon which the device is operative or the power which may be developedat such frequencies.

My invention possesses numerous other objects and features of advantage,some of which, together with the foregoing, will be set forth in thefollowing description of specific apparatus embodying and utilizing mynovel method. It is therefore to be understood that my method isapplicable to other apparatus, and that I do not limit myself, in anyway, to the apparatus of the pres ent application, as I may adoptvarious other apparatus embodiments, utilizing the method, within thescope of the appended claims.

The tube of my invention involves two basic concepts. The first of thesecomprises forming electrode supports of sturdy coaxial metalliccylinders which constitute a radio-frequency transmission line of atleast one and preferably a plurality of quarter wavelength electricallinks, with impedance irregularities at or near certain of the quarterwave points the electrodes themselves forming a portion of thesetransmission lines as considered electrically. Means are preferablyprovided for varying the position of the impedance irregularities toprovide exact tuning, but this is not essential since, as will hereafterbe shown, by a proper combination of the characteristic impedances ofthe quarter-wave sections and their terminating impedances, it ispossible to make the radio-frequency impedance of the supports as viewedfrom the electrodes themselves extremely high, so that the overalleffect is almost as though the electrode capacities together with theinductances required to resonate them were supported freely in the spacewithin an unbroken metallic shield. This latter feature is secured byproviding multiple coaxial line sections forming branch paths of greatlydifferent impedance, certain of these paths acting as lay-passes ofnegli-' gilole impedance at points where it is necessary that'somecirculating currents should flow, although D.-C. insulation must bemaintained, while at the same time maintaining the high impedancedesired in other paths which would otherwise lead to radiation. Byplacing these bypass sections at current nodes, the FR. losses thereinmay be made too small to need consideration.

The second fundamental concept comprises mounting on the ends of suchsupports, preferably in biaxially symmetrical configuration, one or aplurality of cathode-grid combinations which act, as before stated, asthe termini of the transmission lines formed by the supports; mountingthe grid opposed to an anode or other. accelerating electrode in suchmanner as to produce an electrostatic field between grid and acceleratorwhich comprises lines of force very sharply curved in the immediateneighborhood of the grid; and mounting the cathode in the region ofsharpest curvature. One of the best ways of obtaining such a structureis to form the grid of pairs of cylindrical surfaces whose axes areparallel to the plane of the anode, and to make the cathode as afilament or strip having a. flat or slightly concave face lying betweenthe cylindrical surfaces of the grid. This results in the lines of forcefrom accelerator oI' anode normally terminating in the grid structure,none of them reaching the cathode, from which emission is thereforenormally suppressed. A few volts relative change in potential of thecathode, as referred to either accelerator or control grid, results insome of the lines of force from the accelerator terminating in thecathode surface, which is accordingly subjected to an extremely powerfulfield causing very large emission to the anode. The result is what maybe termed an explosive type of emission, giving electron bursts of highdensity for very short periods at the peaks of the cycles. It will beevident that this structure results in a relatively high capacity asbetween cathode and grid, but the tuned transmission line supportenables this capacity to be effectively resonated with an inductance assmall as may be desired and still form a sharply tuned, high Q circuitwhose high resonant input impedance may appear as resis tive, capacitiveor inductive as the conditions of operation may require.

Referring to the drawings:

Fig. 1 is a longitudinal section through a highfrequency oscillator tubeembodying my invention, the particular tube illustrated employing aradial arrangement of filaments and grids.

Fig. 2 is a transverse section of the tube of Fig. 1, showing themultiple coaxial grid-filament supports, and water connections forcooling the filament mounting, the plane of section being on the line2-2 of Fig. 1.

Fig. 3 is a transverse section through the anode structure of the tube,the plane of section being indicated by the line 3-3 of Fig. 1.

Fig. i is an enlarged detailed view illustrating water-coolingconnections from the exterior of the tube to the filament mounting..

Fig. 5 is a schematic sectional view through filaments, control grid,accelerating grid, boundary grid and anode of the tube.

Fig. 6 is a sectional view through the grid support line, showing theradio-frequency by-pass between accelerating and boundary grids and thetuning mechanism for isolating the control grid andwater cooling thesame.

Fig. 7 is an enlarged detailshowing the method of insulating certain ofthe supporting rings upon which the coaxial electrode elements. arecarried.

Fig. 8 is a section taken at right angles to the View of Fig. 1, andshowing the anode-supporting, cooling, and tuning system.

Fig. 9 is a perspective view of the accelerator grid.

Fig. 10 is an elevation of ture.

Fig. 11 is a sectional view, taken on the plane between the filament andgrid structures, and showing in detail the filament support.

Fig. 12 is a fragmentary axial section taken on the line |2t2- of Fig.11.

Fig. 13 is an elevation of the active face of the anode. 7 y

Fig. 14 is a fragmentary section of the anode,

U519 control-grid struc- I advantageous the plane of section beingindicated by the line i l-A l in the preceding figure.

Fig. is an elevation of the boundary grid.

Fig. 16 is a sectional view taken on the line Iii-45 of Fig. 15, andshowing a portion of the anode in elevation.

Fig. 17 is an elevation of one of the filaments.

Fig. 18 shows a modified form of coaxial line structure forgrid-filament support in a tube generally similar to Fig. l, but adaptedfor use either as an amplifier or as an oscillator with inductivefeed-back.

Figs. 19, 20 and 21 are sectional views through the tube of Fig. 18,taken on the lines numbered in accordance with the figures.

Fig. 22 is a longitudinal section through a tube built in accordancewith this invention but wherein a cylindrical, rather than a radialfilamentgrid and anode arrangement is used.

Figs. 23, 24 and 25 are transverse sections through the tube of Fig.22,taken on the lines indicated by the respective numerals.

Figs. 26 and 27 are detailed views indicating the tuning mechanism forthe tube of Fig. 22.

Fig. 28 is a longitudinal section on a larger scale through the filamentsupport of the tube of Fig. 22.

Fig. 29 is a transverse section through the supporting columns of thetube of Fig. 22, showing the construction of the centering mechanism,the

plane of section being indicated by the line 29-29 of Fig. 28.

Fig. 30 is a sectional view taken on the line 38-30 of Fig. 22.

Fig. 31 is a fragmentary section taken on the line 3l-3l of thepreceding figure, showing the passage of the cooling pipe past the anodeand between the two sections of the filament support.

Fig. 32 is an impedance diagram for an openended, half wavelengthsection of transmission line.

In the ensuing specification the invention will first be described inits various aspects as applied to an oscillator tube of moderate power(1. e., approximately 10 KW peak output at 20 to centimeterswavelength). Following this there will be described two modificationsillustrating respectively the application of the principles .of myinvention to a similar tube adapted for amplification or for generationof oscillations by inductive feed-back, and to a somewhat higher outputdevice showing the principles as applied to a tube constructed withcylindrical rather than radial arrangement of electrodes.

The tube shown in longitudinal section in Fig. l is of the demountable,constantly pumped type, as, in fact, are all of those herein describedalthough the principles involved are not limited for use in such tubes.From the structural point of view the tube comprises a series of flangesconnected by sections of tubing and held together in compression. From apractical point of View it is to have the flanges pierced for and heldby circumferential bolts to hold the parts firmly in position when thetube is not under vacuum, but when in use the external air pressuretends to hold the entire device together, and the tube has actually beenoperated without the retaining bolts, these have therefore been omittedin the drawings since they add a further complexity of detail to analready complex structure.

Considering for the moment, therefore, only the external structure whichforms the housing and which supports the remainder ofthe equipment, thetube comprises an anode housing flange l which is grooved to receivetightly the end of a tubular anode housing 2; This housing fits aninternal recess or counterbore in an annular grid flange 3, clamping aboundary grid 4 between the housing tube 2 and the flange 3. A rabbet onthe outer periphery of the flange 3 receives one end of a main supportcylinder 5, whose other end terminates in another annular flange l. Allof the parts thus far mentioned are of metal, and I have found itconvenient to make the flanges of steel, and the tube 5 also of seamlesssteel tubing, while the cylinder 2 may be either chromium or copperplated steel or solid copper, with copper preferred since it forms aportion of a resonating circuit. Carrying on from the flange l is aglass or Pyrex cylinder 9 which abuts a terminal flange it.

As has beenmentioned already, the device as a whole is fully'demountable. The ends-of the tubes contact the flanges with moothmachine fits. The joints thus formed are sealed. by applying theretoordinary wide elastic bands as indicated by the reference characters II, these bands being smeared before application with a small amount ofvacuum line stop-cock grease.

It may be pointed out at this timethat all of the structure thus fardescribed with the exception of the terminal flange i0 is at 11-0.ground potential, and as will later be shown in detail that the entireexterior structure is substantially at radio-frequency ground. Thismeans thatthe insulating section formed by the cylinder 9 is notsubjected to R.-F. fields. It also renders easy the support of thedevice by any desired external means. Part of such support may be theconnection to the pump, which is by a pipe IQ of relatively largeinterior diameter, welded or otherwise secured into the bottom of theanode housing 2. This pipe is not shown in Fig. 1, but is clearlyvisible at the bottom of Fig. 8.

The various elements which contribute to the electronic action of thetube are mounted within the envelope thus formed on columnar supportseach of which has transmission line characterisr tics designed to meetits particular iunction.

These elements are shown in schematic arrangement in Fig. 5, andcomprise an anode 3, a boundary grid 4, an accelerating grid 15, a conrol grid it, and a filamentary cathode ll.

Fig. 5 i drawn to a greatly enlarged scale and shows a fragmentarysection of the elements cooperating with a single filamentary cathode.In the tube here shown six such cathodes are used and the portions shownof the other elements are repeated for each cathode. One advantage ofthe type of structure here shown is that the ability of the tube tosupply power output varies almost directly as the number of filamentused, and that the changes required to add additional filaments arerelatively minor. Tubes have been designed conforming substantially tothe structurev here shown with as high as twenty-four filaments, eachwith its attendant grid-anode structure, but since each of theseassemblies is merely the duplicate of the other as far as performance isconcerned it is sufficient for the present to consider one only.

Considering, therefore, the portion of the elements shown in Fig. 5, theanode l3, preferably made of high conductivity oxygen-free copper, isoperated at the maximum potential of the system, say 10 to 50 thousandvolts positive. It is provided with a V-shaped groove 20 with its axispar.- allel to the axis of the filament. Next, proceeding toward thefilament, is the boundary grid 4,

which i also preferably made of oxygen-free copper. This is providedwith an aperture surrounded by a collar 2| in accurate alinement withthe groove 20 in the anode. Next in line is the accelerator grid IS,with an aperture 22 which is somewhat narrower than the opening in theboundary grid, and which is operated at a potential above the cathode offrom 5 to thousand volts. All potentials mentioned are illu trative andrelative only, since the actual values used will depend upon the size,power output and operating frequency of the device. Furthermore,modifications in design are possible whereby the functions of certain ofthe grid are combined, other electrodes are operated at groundpotential. etc. Such modifications will be considered later; the purposehere is to show the application of the principles of my invention to thepresent tube.

The most important portion of the combination is the arrangement of thecathode-grid structure. The important features here are first, that thecontrol-grid elements comprise parallel cylindrical surfaces, curved asthey are presented to the filament. In the present case they are rods orwires, but formed as the edges of a slot in a flat plate withoutaffecting their performance. Between these surfaces, and slightly backof the plane of their centers of curvature, lies the filament, which hasa flat or preferably a slightly hollow ground face presented to theanode. It is convenient to operate the filament at ground potential(disregarding for the moment the slight voltage drop along thefilament), and, for the powers here considered, to operate the grid I6at 200 to 500 volts negative.

It will be seen that at the orders of voltages given the major fieldsare from the accelerator grid 15 to the control grid. As is, well known,the lines of force constituting such a field terminate at right anglesto the surfaces of the field-defining electrodes. It follows that in theregion adjacent the cathode the lines of force emerge from the gridwires in the general direction of the cathode and then curve verysharply toward the anode in a direction nearly at right angles to theirdirection of emergence. There is also a fairly strong field between thecontrol grid and the oathode itself, which is superimposed locally uponthe field between the control grid and accelerator grid, and is directedtoward, instead of away from the cathode. As a result of the interactionof these two fields none of the lines of force from the accelerator-gridnormally terminate upon the surface of the no tendency to leave thelatter, since the space adjacent it is nearly neutral, with such weakfield as exists therein directed toward the cathode.

As is the case with any grid-controlled tube operated through cut-oil,when the grid swings positive some of the lines of force from theaccelerator-grid which formerly terminated on the control grid nowterminate on the cathode, and as the cycle progresses thecathode-control grid field weakens or even reverses, permitting emissiontoward the anode, and a space charge builds up in the region immediatelyin front of the cathode face which ha the usual effect of limitingemission. The distinguishing feature here is that the region where thefield is weak enough to permit such space charge effect is very shallow,so that even with the low velocities imparted to them by such relativelyweak field the electrons can and do traverse it in a reasonably smallfraction of p a cycle.

they could be cylindrical surfaces cathode. Emission has therefore Thebiasing potential between cathode and grid is so adjusted that emissioncan occur only for an instant at the cycle peaks, and cut-01f may occureven before the first electrons emitted have traversed the space chargeregion. Furthermore, While in this region there is a maximum differenceof velocity as between electrons, both by reason of differing initialvelocities of emission and, more important, by reason of differingacceleration due both to phase of emission and field strength at variousparts of the cathode surface.

The important point is that because the region is so shallow all of theelectrons emitted do get through it before the cycle has advanced toofar, and, having traversed it, fall into the region of high potentialgradient where acceleration toward the anode is very great; space-chargeeffect is or. no further moment, and they receive so large a portion oftheir final velocity that their differences of velocity in the initialregion are immaterial.

It should be realized that while space-charge effect prevents someemission and decreases the acceleration of electrons emitted, it willnot drive -ose which have been emitted back to the oathode nor preventtheir reaching the anode. It follows that the space-charge region may beconsidered as a reservoir for emitted electrons. .With conventionalgrid-cathode structures it is relatively deep, so that, at thefrequencies and powers at which this tube is intended to operate,transit therethrough occupies a major portion of the cycle, and with thevarying velocities obtaining while in this region the electrons stragglethrough to reach the anode in such varying phases that the densitymodulation of the stream is almost if not entirely 10st.

With the arrangement of my invention, how- 'ever, the space-chargeregion is so shallow that .even the stragglers among the emittedelectrons traverse it in less than a quarter cycle and instead of thedensity modulation of the electrons being lost they reach the anode inbursts of such power and suddenness and with such close velocitygrouping that I have termed cathodegrid combinations of this typeexplosive. The object of the design is to make the electron reservoir;constituted by the space-charge region as shallow as possible, and inpractice the ideal can be so far realized as to permit densitymodulation of electron streams at frequencies in the range of 1,500megacycles, where in the past it has been necessary to use velocitymodulation, involving larger and more complicated structures, to getreasonably effective results, even in smaller sizes and at much lowerpowers than those here contemplated.

When the tube here shown is used as an oscillator in the manner now tobe described, the various potentials are so arranged and proportionedthat the transit time of the burst of! electrons is substantiallyone-half cycle. The anode i3 is in a tuned circuit, as is also the grid16. The condition of oscillation then is that the potentials of theanode and the grid swing in the same sense, so that the grid reaches itspeak of positive potential at the same instant as does the anode.

One of the results of the conformation of the electrostatic fields is astrong focusing action upon the electron bursts, and these burstsaccordingly fall upon an extremely limited portion of the anode surface,substantially none reaching either of the intermediate grids. The anodearea upon which the electron bursts impinge is that included in theV-shaped slot 20. The reason for this arrangement is to spread the areaof impact and so increase the size and decrease the intensity of maximumlocal heating, while increasing the cross-sectional area of thermalconductivity by which cooling occurs, and also to insure that secondaryelectrons are not projected into regions of high field intensity whichwould accelerate them so that they, in turn, would cause serious heatingeffects.

It has already been stated that the transit time of the electron burstis one-half cycle of oscillation, and it follows that immediately afterthe electron burst has occurred the anode has started to swing negative.The electrons accordingly reach their maximum velocity at or about theplane of the accelerator grid-ideally, just as they pass the effectiveplane of the boundary grid 4. As the anode continues to swing negativethey encounter a decelerating field, either in an absolute sense or, ifstill being accelerated by the D.-C. field, from the anode at least incomparison to the acceleration of the D.-C. field alone. In passingthrough this decelerating field the electrons are delivering energy tothe anode circuit, and they are traveling at minimum relative velocitywhen they enter the slot 20. This slot acts in some degree as a Faradayspace, and the electrons suffer little change in velocity or energy asthey pass through it. Therefore their work is done and their transittime effectively over as soon as they have entered it.

Since the acceleration of the entire burst of electrons takes place withsubstantial uniformity they retain their close grouping at the time ofimpact, and since the impact occurs when the electrons constituting theburst have suffered maximum deceleration there is a minimum of energywasted as heat and the scillator consequently operates at relativelyhigh efficiency.

For operation in the manner described the desiderata are that thecontrol grid i6 and filament ll should be efiectively isolated from eachother both as regards D.C. and radio-frequency potentials, and shouldhave an efiective capacity sufliciently low so that it may be tuned tothe desired operating frequency, or, in other terms, it must be capableof being connected in circuit with an inductance sumciently small totune to that frequency. The accelerator grid it: must be insulated fromthe other elements to maintain its D.-C'. voltage, but should beeffectively grounded as regards radio-frequency potentials. The boundarygrid 4 should also be grounded to radio-frequency and for convenience inoperation and safetys sake should preferably also be grounded as regardsD.-C. potential, as it is electrically continuous with the envelope. Theanode should be free as regards both A.-C. and D.-C. potentials.

The various mounting and auxiliary means next to be described aredesigned to meet the desiderata as fully as possible. In thisdescription terms such as above or below are used to indicate positionas shown in Fig. 1. They have no other significance, as the device maybe operated in any'position.

Starting at the bottom of Fig. l, with the flange Ill, a highconductivity column or pipe 25 extends a major portion of the length ofthe entire device to the plane of the flange 3 and the boundary grid 4.This column is brazed or otherwise permanently secured into the flangeif) so as to be accurately concentric with the remainder of the tubestructure and, of course, to be vacuum tight. At its upper end it isthreaded to receive a grid-support ring 21, which is clamped betweenlocking nuts 29 and 3t, and an additional locking screw 3i (Fig. 10) isalso provided for further security. The pairs of parallel grid wires itproject from the ring 2'! parallel-to its radii, six pairs of grid wiresbeing provided in the present design, the pairs being equidistantlyspaced around the periphery of the ring.

Two sliders are mounted on the column 25. The upper slider 28 comprisesa short section of tubing 32 surfaced to a sliding fit on the column 25and shouldered at each end to receive discs 33 and 3d between which ashort section of tubing 35 is clamped. The column 25 is provided in thisregion with a longitudinal slot for the passage of a screw 81 whichengages a piece of tubing 39 sliding within the column. The tubing 39terminates in an annular block 4%], and an adjusting rod All is threadedinto one side of the block and passes to the exterior of the'tubethrough a gland box 32 and a Wilson seal 53. It is apparent that theposition of the slider may be adjusted by sliding the rod 4!.

A word as to the Wilson seal may here be in order, and in thisconnection attention is drawn to the showing at the lower right of Fig.8. The seal proper consists of a normally flat washer 45 of synthetic ornatural rubber, which is forced against a conical seat 41 by theinternally conical edges of a gland Q9. When the washer is unstressedthe aperture therethrough is slightly too small for the rod 50 which itis desiredto seal. The seal is lubricated with a small quantity ofvacuum stop-cock'grease. Such a seal is vacuum tight under conditionswhere other known types of packing would leak badly, since thedifferential pressure on the washer serves to make it hug the centralrod more tightly and'it remains tight whether the rod be subjected torotary or sliding motion in either direction.

Returning to the general tube structure, the second and lower slider 5|is essentially similar in construction to that just described, exceptthat its actuating rod 52 is mounted externally of the column 25 throughthe gland box 52 and Wilson seal 53.

The slider 55 makes a close sliding fit within a cylindrical conductor54 mounted in the flange l6 accurately concentric with the column 25 andmaintained in this concentric relation both by the slider 5| itself andby an auxiliary diaphragm or spacer 55. The tubing 54 does not extendthe full length of the central column, but terminates a distance abovethe flange l which is somewhere in the neighborhood of one-eighth of awavelength at the mean frequency for which the tube is designed.

Accurately coaxial with the column 25 and its surrounding conductor 5!is a third conducting cylinder 51, mounted on the flange l and extending below it for approximately one-eighth wavelength, so that thetwo conductors 54 and 51 overlap by a distance approximately equal toone-quarter wavelength of the average operating frequency of the device,wavelengths in this sense being used to mean the wavelength of thefrequency transmitted along the two tubes as a coaxial transmissionline. There is no metallic contact between the two conductors 5d and 51,and they are separated by vacuum so that dielectric loss does not occurin the space between them.

Column 51 is brazed or otherwise secured in the flange I, is made ofhighly conducting material (preferably oxygen-free copper) and ispreferably provided with a cooling system comprising a water pipe 50coiled around and soldered to at the right of Fig. 1. v

The upper end of the column carries an intermediate ring 6| whichsupports indirectly one end of each of the filaments ll. The other endsof these filaments are carried by a group through a clamping cap 61,

not subject from radio-frequency fields.

The actual filament mounting can best be seen in Figs. 11 and 12. Eachends of the filaments are clamped in an annular groove 13, mounting ring14 51 by the interupper end of the lugand extend for approximatelyonewith its supporting rings 61 and M and the group of filament supporttubes 62 is the filaments themselves.

their expansion.

Each filament is preferably formed of round tungsten wire, one surfaceof which is ground slightly concave. The diameter here used is 50 mils.The grinding is preferably performed in a jig which deforms the wireslightly in the longitudinal direction, so that the ends of the filamentare ground a few thousandths of an inch thinner than is the centralportion. This grinding forms the flat emitting surface of the filament,and if done with a relatively small wheel whose axis is maintainedparallel to the length of the filament, it gives the slight hollowgrinding which has already been stated to be advantageous. The efieot ofthinning the two ends, adjacent the point where the filament is clamped,is to give a greater current density at these points, with a consequentgreater liberation of heat which compensates for the heat conduction tothe clamping means and results in substantially constant temperature andsubstantially constant emission over the entire eifective length of thefilament.

occur in practice.

Cooling for the support of the inner ends of the filaments isaccomplished by conduction system within the support pipes 62themselves. A small water pipe end of the lug. From there it returnsthrough the pipe 62 externally of the pipe 99 to the bottom of pipe 62,wher the end 99' of the next pipe is connected to carry the water to thenext filament support, circulation thereby occurring through each of thesupport pipes 62 in succes- S1011.

The supply for this circulatory system is through a fitting designatedby the general refcomprising coaxial pipes 92 manently secured to thesupport ring 63 (see 9| passes through the flange 7 and is insulatedtherefrom by insulating bushor other refractory between constitute theconducting system for supplying the filament current. The return circuitis through the column 51 and the flange 7, to which a second connectinglug (not shown) is attached.

There are two other features comprised within passing through 2. Wilsonseal :62 in the gland box 42. The second is a cooling pipe I03 whichextends substantially the full length of the inner column 25 and issoldered thereto adjacent its upper end for better heat transfer.

We are now in a position to consider the electrical characteristics ofthe filament-grid structure in view of the desiderata above and it isbelieved appropriate to do this point, since the same principles areinvolved in the supports for the remaining elements of the device andthe explanation of all will be simplified if these principles are inmind. The necessary separation of the elements as regards D.-C. or lowfrequency potentials have already been accounted for. There is nometallic connection between the grid-support column 25 and thefilament-support system comprising the column 51, and the support pipes6.2. Remaining to be accounted for is the impedance relationship betweenthe grid and filament members, and this is dependent upon the impedanceof the coaxial transmission line formed by the inner and outer columns25 and 51 and the coaxial conductors associated therewith.

The impedance characteristics of transmission lines whose lengths are ofthe same order of magnitude as the wavelengths of electricaloscillations transmitted thereby are now well known, but they arerestated here for convenience in the explanations that follow. Most oithem can be derived from the impedance diagram of a halfwave line openat the output end, as shown in Fig. .32, which indicates such a linediagrammatically, and shows the approximate curve of relative impedancelooking into any portion of the line from the right, resistance of theconductors themselves being assumed to approach zero. Extremelyshortsections show a high capacitive reactance, which falls to thecharacteristic impedof the line at the /8 wavelength point, and to zeroat the quarter-wave point, i. e., a quarterwave open-ended line acts asa dead short. From this point on the apparent reactance is inductive,rising again to the value at the point and approaching infinity at i.

The same diagram may be taken as representing the impedance of ashorted-end line if the origin be taken at the nodal or quarter-wavepoint, which appears as a the line. For short sections the reactance issmall and inductive, it rises to at the /8)\ point and approachesinfinity at Since this appears as an open the length of the line repeatsthe portion of the diagram shown at the left of the nodal point.

Stated in another manner, a quarter-wave open line or a half-waveshorted line appears much like a series resonant circuit, while aquarterwave shorted line or a half-wave open line appears like ananti-resonant or parallel resonant circuit.

The only other relationship necessary to the understanding of thepresent invention is that the characteristic impedance of a quarter-waveline is the geometric mean between closing impedances. The short-circuitand opencircuit, increasing short when looking into its input and comesthe closing impedance of circuit conditions are, of course, merelyspecial cases of this general relation.

The lines comprising the element supports of the tube of my inventionmay be considered from a number of aspects, all depending on the generalrelationships above set forth, but in the treatment here adopted theyare generally considered as divided into sections of quarter-wavelength, or thereabout, as this is believed to lead to the simplestexplanations.

We are interested in the impedance of the grid-filament support line asviewed from the grid end, but this impedance is dependent upon theterminating or output impedances of the various sections, and therefore,in order to determine what the grid-end impedance will be, we mustconsider the various elements, section by section, starting from theoutermost or lower end of the tube as shown in Fig. 1.

From this aspect the first section of the structure is the sectionincluding the adjusting rods 52, M, etc., the flange l0, and the sectionof the tubular conductor 54 illustrated as below the end of the column51. Electrically this portion of the structure is a single conductor,and viewed from its upper end constitutes an end-fed antenna. It ispreferable that its length be of the order of one-half wavelength at theoperating frequency of the device, in which case its effective impedanceZ2. will be in the neighborhood of 1,000 ohms. If its length be reducedto one-quarter wavelength its efiective input impedance will likewise bereduced to the neighborhood of from 50 to ohms, the quarter wavelengthcondition being the least desirable in practice. This antenna isconsidered as being fed by the coaxial transmission line comprising thetubular conductor 54 as the inner element and the column 51 as the outerelement. With the spacing shown such a transmission line will have acharacteristic or surge impedance Z0 of about 10 ohms, and as hasalready been stated the length of this section of transmission line isapproximately dition to be fulfilled exactly the impedance looking intothe coaxial line from the grid end will be If the antenna section of thesystem have an impedance of the order of 1,000 ohms, the characteristicimpedance of the line being 10 ohms, the input impedance of the linewill therefore be of an ohm. This low impedance therefore bethe sectionof line immediately preceding it. From one point of view it acts as aradio-frequency by-pass between the inner conductor 54 and the outerconductor 51, so that viewed from the input end,

at radio-frequencies the cylinder 54 and outer column 51 appear as asingle conductor, and form, in connection with inner column 25, a singleradie-frequency transmission line considered as fed from thegrid-filament end through a slight impedance irregularity where theinner cylinder 54 terminates.

will be considered later.

Even if the conditions as to impedance of antenna and length of thecoaxial line constituting the column 51 and cylinder 5d are not exactlymet the result will be substantially the same. The

Its effect from another point of view the outer line and 1 B(practically) the energy antenna impedance can easily be kept above 100sulator), and the succeeding sections can be ohms, making the impedancelooking into the treated as if they terminated at this point in anquarter wavelength line 1 ohm. If the length of infinite impedance. Itshould be noted, however, the linesection is not exactly one-quarterwave, that at t e frequencies we are considering subbut is stillmaterially greater than one-eighth 5 stitutio-n of an insulator for theline sections wavelength, the input impedance will still be low woulddrop the impedance to a finite value and in comparison with thecharacteristic impedance introduce large losses through radiation. anddiof the line, and although more power will escape electric phenomena.

than if optimum conditions are met the amount The design problem to bemet, therefore, is the of power wasted by such undesired radiation will1 design of a structure which, when terminated by be very small. animpedance approaching infinity, will have the The section of the innerline comprising the properties of antiresonant circuit as viewed fromcylinder 55 and column 25 terminates in the cathode and grid. Thisstructure is provided by slider 55, which, as it is of large area andmakes two additional quarter-wave sections of the same good contact withboth conductors, may be conline.

sidered as of zero impedance. This section may The first of thesesections extends to include be tuned to exactly one-quarter wave bymoving the upper slider 28, and its design is such that the slider. Dueto the spacing between the two its electrical length may be changed inopposite conductors the characteristic impedance of this sense to itsphysical length; i. e., such that it section of transmission line ishigh, and the may be fitted in beneath the section above it resistanceof the line were zero the input impedeven when the length of the uppersection inance would be infinite. Actually it may always be creases withdecreased frequency of operation or made to exceed 180,000 ohms andunder optimum vice versa.

conditions may reach ten times this value. This This efiect is obtainedby means of the irregusection therefore forms a tuned radio-frequencylarity introduced by the low-impedance line seechoke of extremely highimpedance interposed tion constituted by the slider 28. From the topthat practically all energy reaching it is reflected ance line of lessthan ,4; wavelength which thereback to its source. fore appears as acapacity variable from zero What actually happens can be expressed moreto some small value as the slider is moved to nearly in the terms of lowfrequency power line change its length from zero toward which is fed bya line terminating immediately A above the top of the column 54. Currentfedto Ain skin effect none will flow transversely through the wall ofthe conductor. In so flowing the current meets an enormous impedancesay100,090

distance between the slider and the node will vary with frequency, ofcourse, but only slightly with the position of the slider.

It has already been pointed out that the node is eiiectively equivalentto a short-circuit, and hence, since by moving the slider we may moveditions are met.

From still a slightly different aspect, the small and largely resistiveimpedance offered by the Outer lmeris at a current node We thereforethat we have an elastic or extensible quarterwave section of line.

The final or grid-filament section may thus be resonated or otherwisetuned to give optimum operating conditions. In the case of Fig. 1, wherecapacity feed-back between anode I3 and grid cap 99 is used, the desiredtuning of this section must provide a capacitive reactance. This is obtained by making the grid-filament section slightly longer than one-halfwavelength or, in other this case being the apparent input impedance ofradiated.

From whatever aspect the matter be considered the resul is the same: Thesections of the transmission line above the current node terminate inord r y ood 111511131301. There is some at grid and filament it presentsa small antisumption of power, which can be neglected in resonantcapacitive reactance. Under these cirfurther consideration (as in thecase of the inoumstances the filament-grid system appears as a capacityin series with the capacity between the grid structure and the anode,and this latter capacity is adjustable by varying the position of thecap 99. When, therefore, the potential of the anode swings, the gridwill assume a potentialwith respect to the filament (and ground) whichis intermediate between cathode and anode potential, and which bears theproportion to the total potential between anode andfilament that theeifective series capacity between anode-grid and grid-filament bears tothe apparent capacity between filament and grid. In other words, thearrangement is "essentially a capacitive voltage divider which swingsthe grid potential in the same sense that the anode potential swings,and in fixed and predetermined proportion thereto. Since the criterionfor oscillation of the device is that the grid and anode should swing inthe same sense and in step, the result is a highly effective capacityfeed-back which is under control either by varying the actual capacitycoupling with the cap 99 or by varying the effective resonant inputcapacity of the grid-filament circuit by varying the position of theslider 28.

By the use of the two sliders the device is thus given its veryconsiderable tuning range. The lower slider brings the current node tothe point where the antenna is fed; the upper slider 28 moves the nodalpoint immediately above it and thus tunes the filament-grid section. Theactual point of importance is that by adjusting the position of thesliders the efiective resonant impedance of the filament-gridcombination may be made to assume any value which may be desired, sincethe node above the slider 28 may be moved near enough to the ratherlarge lumped cathode-grid capacity to embrace between the node and thatcapacity the exact small line inductance required for tuning it. Inactual practice the effective impedance will be made capacitive andsmall in comparison with the physical grid-cathode capacity, but itmight, if desired, equally well be made inductive or resistive.Furthermore, since the efiective resistances in the circuit areextremely low, and the losses are also small even though the circulatingcurrents may be large. t 1

A system of transmission lines, chokes and bypasses similar to that usedin the filament-grid circuit is employed across the filament to preventtransmission of energy to D.-C'. insulation and to prevent filamentdamage by R.-F. currents. The actual ground point on the filamentcircuit is the fiange 1 on the outer casing 5 of the device. This,however, is unimportant and the efiect of the transmission linearrangement may be considered as though the ground point were at theinner end of the filament. This may be considered as terminus of aquarter wavelength coaxial transmission line comprising the tubularconductors 19 and 80, which is open at its lower end, terminating in ahigh impedance. The transmission line impedance is again low, beingofthe order of, say, 5 ohms, and the line therefore forms a negligibleseries impedance as before, acting as a by-pass to the inner conductor.This again is a quarter wavelength line with the pipe 62 as its innerconductor, terminating in a dead short, and therefore offering very highimpedance. As the potential imposed across this impedance is merely thatwhich can build up across the short filament, amounting to a few voltsat most, the escape of power through the filament support may beneglected, and the high impedance effectively in series with thefilament prevents circuring to surround the and emerging through theflange lation of R.-F. currents which might otherwise cause hot spotsand burn-outs.

We are now ready to consider th mounting of the remaining elements, i.e., the accelerator grid, boundary grid, and anode, which elements areshown in elevation in Figs. 9, l5 and 13 re spectively. The acceleratorgrid is mounted from a side tube :05, which is welded to roject throughthe wall of the housing 5 immediately below the flange 3. This sidetubecarries at its outer end a fiange lll'l which is surfaced to receivethe tubular glass insulator I09, and the latter, in turn, carries aterminal flange I ID. This structure may best be seen in the enlargeddetail view of Fig. 6. As in the case of-the main tube envelope, thetie-bolts which hold the structure together are omitted, but it will beunderstood that it is assembled in the same envelope with groundsurfaces reenforced by greased rubber bands or gaskets l the seals. Twotubular conductors are fixed to and project inwardly from flange H0. Theinner conductor H2 is spaced from the outer conductor H3 and is heldaccurately concentric therewith by an annularspacer I M. "Theaccelerator grid I5 is supported from the inner member by atubular'bracket ll5,-the end of which ts within the conductor H2 and isrigidly secured thereto. 'A cooling pipe Ill, bent into a acceleratorgrid, has its ends brought down parallel to the support bracket I I5 andenters the innerconductor on either side thereof, the ends of the pipepassing into the interconductor space distally of the spacer H4 llll. Atuning slider H9, which nearly fills the space between the inner andouter conductors and does not make actual contact therebetween, isoperated by means of a hook I20 whose end projects through alongitudinal slot in the conductor 2. A control rod l2l is threadedtothe end of the hook and emerges through a Wilson seal I22.

Th supporting bracket H5 and cooling tubes In are carried up to theinterspace between the control grid and the boundary grid through an a glar fitting or shield I25, which passes through a notch 12! cut in oneside of the filament support ring 6 I. This construction is shown inFigs. 1, 6 and 11, each of these figures showing sections of the shield.The shield is electrically continuous with a pan I29 overlying andcontacting the filament support ring 14 and slotted immediately abovethe filaments, which forms an additional shield or barrier to separatecompletely the anode and control-grid sections of the tube except at thepoints where intercommunication is necessary or desired. The shield andpan therefore form one terminus, and the accelerator grid and coolingpipe ll'l form the other terminus of the radio-frequency transmissionline comprising the slige tube )5 and the tubular conductors H2 and Fromwhat has gone before it is believed that the operation of thisarrangement will be readily apparent. Again we have an antenna systemcomprising the control rods l2! and cooling tubes Ill, plus theprojecting end of the conductor H3, which is fed by and offers arelatively high impedance to a quarter wavelength transmission line of10w impedance formed by the side tube I05 and the conductor H3, andthere is accordingly a radio-frequency by-pass' between the groundedouter case 5 and tube H15 of the conductor H3. Within'this there fashion'as is the main I I which form the control grid is another seriessection of prising the conductors It? and H3 andterminating in a shortformed y the spacer I14 This inner line is tuned to a quarter wavelengthby means of the slider I20, which acts as a loading capacity andincreases greatly the electrical length of the line, Inv practice thisslider is moved back to a point frorn which the line ap pears as a verylarge inductance at the operating wavelength. The proper point is thatat to the shorting spacer II 4; very high impedance at the shield wherethe grid I5 and are suppprted, and preventing any appreciable powerbeing transmitted past this point to be radiated. The capacity of thegrid I5to the boundarygrid 4 is large, and that to I5 is small; there islittle coupling tending to swing the accelerator grid I5, and itconsequentlytends to maintain very nearly zero R.-F. potential. V p

e As has already been described and as shown in detail in Fig, 16 theboundary grid 4 is firmly clamped between the flange 3 and the anodehousing 2, and is therefore physically and definitely at the resonantline is one-half wavelength long, and may b considered as terminatingbetween the of the'anode, i. e., at a potential node, so that there islittle tendency for power to escape from the support structure. Suchtendency as there is for. power to leak from' the support point issuppressed by either .or both of two methods. the cases where the tubesoperat at a fixed wavelength, is a movable plate I32 mounted on the Theside tub carries a. metallic flange I M, with a glass insulator tube I42fitted against it and in turn carrying a terminal flange I43.

transmission line com:

;1 ro rn the electrical point of view the anode body is a simplecylinder with closed ends. Its complexity, as shown in Fig. 8, is dueprimarily to the provision for circulating cooling water Within it, andto the provision of whatmay be termeda rough tuning" device. I 1; vOwing to the necessity for providing cooling, the body itself must bewater-tight, andaccorde ingly it is constructed of a flared cylinderI47, to the flared end of which the anode faceiIS is; hardsoldered.Theother end of the cylinder is closedby a threaded disc I49. r p p I I;

The supporting pipe I44 enters the flared cylinder I41 thrcughan aperturin the side there of. The end of the pipe is threaded into a boss I48onan inner ballle cylinder I50,

The boss {48 extendsinternally to form a cylin drical chamber I5I, whichconnects by a sidepipe I52 through the end I 53 of the baffle cylinder,so;

that water introduced through the pipe I44 is discharged directlyagainst the active face I3 of the anode, and thence is forced around theexterior of the baflie cylinder to reenter its open end. It can thenreturn within the cylinder to enter the open end of a return pipe I54,which is mounted concentrically Withinthe pipe I44 by means of aperforated cap I55 which fits over the end of thepipe out through thecompresses a rubbergasket I 4.5, sealing the joint between the pipe I44and the anode;b0dy to make it water and vacuum tight. v p

The upperend of the pipe I 54 is centered in.

I 59, and its course can be traced by the arrows in the drawings throughthe outer pipe, the perforations in the cap I 55, the side pipe I52, andthence around the bafiie cylinder I50 and back through the central pipeI54.

The action of the mounting follows the principles already set forth,although the application is somewhat dii ferent. A disc I 69 is con.-nected to the flange I4I both electrically and mechanically, and carriesa cylinder IGI. The pipe I and the'cylinders I 4i] and Nil form atranswavelength long. Electrilating cylinder I42, which must withstandthe fullD. -C. anode potential of 20,000 volts or more. The length ofthis section is measured from the anode and its housing, and theimpedance at its high, so that looking into it from the anode theimpedance is also very high.

This high impedance is connected in shunt across the lineformed by theanode body I30 and ance path to the outer world.

In other terms, the full wave line is connected so near the node ofthemain anode oscillator circuit that only a. few volts are effectiveacross its termini, andtherefore very small currents will tend to flowtherein, representing a power loss of where V is the small input voltageand Zthe I44, its lower end passing discharge chamber I 5I. The cap.

